This was not, so far as i know, explicitly defined in the past. Different browsers are free to implement event ordering however they see fit. While most are close enough for all practical purposes, there have been and continue to be some odd edge cases where browsers differ somewhat (and, of course, the many more cases where certain browsers fail to send certain events at all).
That said, the HTML 5 draft recommendation does make an attempt to specify how events will be queued and dispatched - the event loop:
To coordinate events, user
interaction, scripts, rendering,
networking, and so forth, user agents
must use event loops as described in
this section.
There must be at least one event loop
per user agent, and at most one event
loop per unit of related
similar-origin browsing contexts.
An event loop has one or more task
queues. A task queue is an ordered
list of tasks [...]
When a user agent is to queue a task,
it must add the given task to one of
the task queues of the relevant event
loop. All the tasks from one
particular task source must always be
added to the same task queue, but
tasks from different task sources may
be placed in different task queues.
[...]
[...]a user agent could have one task queue
for mouse and key events (the user
interaction task source), and another
for everything else. The user agent
could then give keyboard and mouse
events preference over other tasks
three quarters of the time, keeping
the interface responsive but not
starving other task queues, and never
processing events from any one task
source out of order. [...]
Note that last bit: it is up to the browser implementation to determine which events will be grouped together and processed in order, as well as the priority given to any particular type of event. Therefore, there's little reason to expect all browsers to dispatch all events in a fixed order, now or in the future.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
